Concrete beams or purlin supports are becoming common in building construction where materials and casting yards are locally available. The use of precast concrete beams or purlins is in many areas becoming competitive with steel or wood. The free span of the beams may vary widely from say 5 to 50 meters, as do the size and dimensions of the cross section. For weight, stiffness and strength reasons, the beams are usually always in the form of an I. That is, they have a vertical stem or web and top and bottom flanges. The flanges, and particularly the lower flange, usually have the highest concentration of steel reinforcing and some of such steel may be quite close to the surface of the concrete.
Such precast beams used to be significantly overdesigned with an excess of steel and dimension. However, to reduce weight and to be competitive with comparable span steel and wood beams, slender more carefully designed precast concrete beams are employed, and such beams are not always capable of having holes or anchors inserted into them without compromising the integrity and design strength of the beam. More and more by code or regulation, drilling or inserting fasteners into steel reinforced concrete beams is forbidden. This is particularly true in the lower flange which may have a high concentration of critical steel close to the concrete surface. Moreover, the precise location of the steel is not always known, and because of tolerances involved in the casting process may vary somewhat from beam-to-beam, even though of identical exterior dimension and design capacity.
In the past, some fastening techniques have utilized a hole through the web in the area of the neutral axis of the beam. Such techniques may involve angle brackets fastened to the web through the hole and projecting laterally from the beam. Other systems simply wrap metal strap through a hole or slot in the web around the lower flange. Others may use a large inverted U-bolt extending through the web hole to hold a channel or strut to the bottom of the beam, from which the load is suspended. In the lower flange, it was common to drill a hole and insert an anchor to secure a channel or rail which in turn suspended the load. These methods, now mostly prohibited or frowned upon because of possible compromise of beam strength, were also messy and labor intensive to accomplish properly. Even if the holes or systems are precast or cast in place, this adds to the cost of the beam and even so they usually require cleaning or removal of concrete intrusions.
Also, the hole, whether in the web or lower flange, once formed has no adjustability longitudinally of the beam. Such holes, particularly in the web, negatively affect the appearance of the beam. More importantly, such holes, particularly in the bottom flange, introduce local forces where the beam is most vulnerable.
A variety of hanging systems avoid the beam hole problem by completely encircling the beam. This can be done with a perforated metal strap or strip completely around the beam, or with a large inverted U-bolt or threaded rod going over the top of the beam, and holding a channel to the bottom of the beam from which a load is suspended Another system may use parallel struts, one over the top of the beam and one below the beam, such struts being interconnected with threaded rod on each side of the beam. Such threaded rod usually requires a multiplicity of nuts and washers and they have to be adjusted or tightened evenly. These systems are useful only if the area over the beam is accessible, and even then require a large number of parts and adjustments. The latter is true since the parts involved are not readily adjustable other than by cutting to obtain an optimal fit for different size or shape beams.
Because of the shortcomings of the above systems, there has evolved some tailored systems designed to grip only the exterior of the bottom flange. These systems are relatively costly and have a limited range of beam sizes and configurations on which they will work. The simplest system uses a horizontal strut which extends beneath the beam. Brackets are mounted to extend from the top of the strut at each edge of the bottom flange. The brackets terminate in inwardly inclined upper ends which extend over the top of the edges of the beam bottom flange. The brackets, however, usually fit only one size or type of beam and problems are encountered if the flange dimensions may vary even slightly longitudinally of the beam. The load is usually suspended from the strut and special holes in the strut may be required for the brackets, and certainly the right size and shape brackets. Again, multiple nuts, bolts and washers are required. The hanging system is difficult to fit securely and snugly. The brackets and the spacing of the brackets still have to be tailored for a particular beam or a very narrow range of beams.
In a steel beam, clamps are used over the edge of the bottom flange, but the configuration and size of concrete beams makes application bulky, limited and awkward.
A more complex tailored hanger is that sold by SINARD and known as the REF-500. It includes two main brackets horizontally adjustable by use of a thumb screw extending beneath the beam. Each bracket includes a short vertical leg. At the top of each leg is pivoted a short obtuse angle double roller bracket. The lower roller is designed to contact the side or edge of the flange forcing the upper inwardly offset roller to pivot over the top of the flange until it contacts the sloping upper surface of the lower beam flange, all as the two main brackets are tightened about the flange with the thumb screw. Such hanger is relatively complex and expensive and depends on the strength of both the main and roller brackets for its load capacity Moreover, its range of applicable beams, sizes and shapes is limited.
It would, accordingly, be desirable to have a simplified universal beam hanger which does not have to circle the beam completely, which does not need a hole or strut in the beam, and which has wide adjustability to accommodate a wide variety of beam sizes and shapes and which can be used on both concrete and steel beams. It would also be desirable to have such a hanger using few low cost and easy to install, assemble, and operate parts.